Method for removing cationic dyes from an aqueous solution using an adsorbent

ABSTRACT

A method of removing at least one cationic dye from an aqueous solution. The method includes contacting the aqueous solution with an adsorbent comprising a water-insoluble membrane disposed on a substrate. The water-insoluble membrane comprises cross-linked humic acid, at least one alginate, and hydroxyethyl cellulose. The contacting forms a treated aqueous solution having a lower concentration of the at least one cationic dye relative to the aqueous solution.

BACKGROUND OF THE INVENTION

Technical Field

The present disclosure relates to the field of methods for removingcationic dyes from an aqueous solution. More specifically, the presentdisclosure relates to a method of removing one or more cationic dyesfrom an aqueous solution using an adsorbent comprising a water-insolublemembrane comprising cross-linked humic acid, at least one alginate, andhydroxyethyl cellulose disposed on a substrate.

Description of the Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, is neitherexpressly nor impliedly admitted as prior art against the presentinvention.

Dyes are commonly used in textile, pharmaceutical, food, tanning andpaper industries (See M. N. Ashiq, M. Najam-Ul-Haq, T. Amanat, A. Saba,A. M. Qureshi, M. Nadeem, Removal of methylene blue from aqueoussolution using acid/base treated rice husk as an adsorbent, Desalinationand Water Treatment, 49 (2012) 376-383. M. T. Yagub, T. K. Sen, S.Afroze, H. M. Ang, Dye and its removal from aqueous solution byadsorption: a review, Adv. Colloid Interface Sci., 209 (2014) 172-184.J. R. Deka, C. L. Liu, T. H. Wang, W. C. Chang, H. M. Kao, Synthesis ofhighly phosphonic acid functionalized benzene-bridged periodicmesoporous organosilicas for use as efficient dye adsorbents, J. Hazard.Mater., 278 (2014) 539-550, each incorporated herein by reference intheir entirety). Types of dyes include basic and or cationic dyes whichare positively charged stains that react with a material that isnegatively charged. Cationic dyes typically contain amino groups, oralkylamino groups, as their auxochromes. Examples of cationic dyes aremethylene blue, rhodamine B, crystal violet, basic fuchsin safranin,pararosaniline, etc.

Waste water containing dyes has resulted in pollution of many watersources, including ground water and river water from which tap water anddrinking water are obtained. Dyes affect the chemical oxygen demand(COD) and sunlight penetration in water, both of which have adetrimental effect on aquatic life (See K. Zhou, Q. Zhang, B. Wang, J.Liu, P. Wen, Z. Gui, Y. Hu, The integrated utilization of typical claysin removal of organic dyes and polymer nanocomposites, Journal ofCleaner Production, 81 (2014) 281-289, incorporated herein by referencein its entirety). Cationic dyes have such a high color intensity thateven at trace levels, they impart color to water, making it undesirablefor consumption. Although cationic dyes such as methylene blue (MB) andrhodamine B (RhB) are not as hazardous as azo or reactive dyes, acuteexposure to them may result in serious health issues. Inhalation of MBcan cause increased heart rate, and ingestion of MB may lead tovomiting, nausea, jaundice, tissue necrosis and quadriplegia (See A.Gürses, A. Hassani, M. Kiran

an, Ö. Açi

li, S. Karaca, Removal of methylene blue from aqueous solution using byuntreated lignite as potential low-cost adsorbent: Kinetic,thermodynamic and equilibrium approach, Journal of Water ProcessEngineering, 2 (2014) 10-21, incorporated herein by reference in itsentirety). RhB is a common staining dye in biotechnology known to haveneurotoxicity and carcinogenicity, and can cause irritation of the skin,eyes and respiratory tract (See H. Mittal, S. B. Mishra, Gum ghatti andFe₃O₄ magnetic nanoparticles based nanocomposites for the effectiveadsorption of rhodamine B, Carbohydr. Polym., 101 (2014) 1255-1264. K.G. Bhattacharyya, S. SenGupta, G. K. Sarma, Interactions of the dye,Rhodamine B with kaolinite and montmorillonite in water, Appl. ClaySci., 99 (2014) 7-17, each incorporated herein by reference in theirentirety).

Because dyes are toxic and non-biodegradable, removal of dyes from watersources is necessary to provide clean and safe water and protect aquaticlife (See N. Jain, A. Bhargava, J. Panwar, Enhanced photocatalyticdegradation of methylene blue using biologically synthesized“protein-capped” ZnO nanoparticles, Chem. Eng. J. (Lausanne), 243 (2014)549-555. Y. Li, Q. Du, T. Liu, J. Sun, Y. Wang, S. Wu, Z. Wang, Y. Xia,L. Xia, Methylene blue adsorption on graphene oxide/calcium alginatecomposites, Carbohydr. Polym., 95 (2013) 501-507, each incorporatedherein by reference in their entirety).

It is an object of this disclosure to provide methods for removingcationic dyes from water or an aqueous solution using an adsorbentcomprising a water-insoluble membrane comprising cross-linked humicacid, at least one alginate, and hydroxyethyl cellulose disposed on asubstrate.

BRIEF SUMMARY OF THE INVENTION

The present disclosure relates to a method of removing at least onecationic dye from an aqueous solution. The method includes contactingthe aqueous solution with an adsorbent comprising a water-insolublemembrane disposed on a substrate. The water-insoluble membrane comprisescross-linked humic acid, at least one alginate, and hydroxyethylcellulose. The contacting forms a treated aqueous solution having alower concentration of the at least one cationic dye relative to theaqueous solution.

In one or more embodiments, the weight ratio of humic acid:at least onealginate:hydroxyethyl cellulose lies in the range 5-30:40-90:5-30.

In one or more embodiments, the at least one alginate comprises anunmodified alginate, a modified alginate, or a combination thereof.

In one or more embodiments, the humic acid, the at least one alginate,and the hydroxyethyl cellulose are cross-linked by at least onecross-linking agent selected from the group consisting of aldehydes,oxidoreductase enzymes, and a combination thereof.

In one or more embodiments, at least one of the cross-linked humic acid,at least one alginate, and hydroxyethyl cellulose comprises or ismodified to comprise a first molecular moiety, and the substratecomprises or is modified to comprise a second molecular moiety, and thedisposition of the water-insoluble membrane on the substrate comprisesbinding of the first molecular moiety to the second molecular moiety.

In one or more embodiments, the pH of the aqueous solution ranges fromabout 3 to about 10.

In one or more embodiments, the substrate comprises at least oneselected from the group consisting of polypropylene, polystyrene, PET(polyethylene terephthalate), polyimide, PEN (polyethylene naphthalate),agarose, acetate cellulose, PC (polycarbonate), glass, plastic, rubber,a metal, an alloy, a ceramic, a carbonaceous material, a polymer, sand,silicon, and silica.

In one or more embodiments, the at least one cationic dye is selectedfrom the group consisting of methylene blue, rhodamine B, crystalviolet, basic fuchsin safranin, pararosaniline, and a combinationthereof.

In one or more embodiments, the at least one cationic dye is methyleneblue, and the adsorbent removes at least 90% of the methylene blue fromthe aqueous solution.

In one or more embodiments, the at least one cationic dye is rhodamineB, and the adsorbent removes at least 90% of the rhodamine B from theaqueous solution.

In one or more embodiments, the at least one cationic dye is rhodamineB, and the concentration of the rhodamine B in the aqueous solution isadjusted to be below 75 mg/L.

In one or more embodiments, the method further comprises regeneratingthe adsorption ability of the adsorbent.

In one or more embodiments, the regenerating the adsorption ability ofthe adsorbent comprises treating the adsorbent with at least one mineralor strong acid and/or at least one organic acid for a period of timeeffective to desorb the at least one cationic dye from the adsorbent.

In one or more embodiments, the concentration of the water-insolublemembrane of the adsorbent contacting the aqueous solution ranges from0.04 g/100 ml to 0.64 g/100 ml of the aqueous solution.

In one or more embodiments, the adsorbent is disposed in a fixed bedreactor or fluidized bed reactor and the contacting involves passing theaqueous solution through the fixed bed reactor or fluidized bed reactor.

In one or more embodiments, the fixed bed reactor comprises a cartridge.

In one or more embodiments, the cartridge further comprises activatedcarbon.

In one or more embodiments, the adsorbent has a form selected from thegroup consisting of a granule, a pellet, a sphere, a powder, a wovenfabric, a non-woven fabric, a mat, a felt, a block, and a honeycomb.

In one or more embodiments, the aqueous solution is contacted with theadsorbent at a temperature of about 10-90° C. and a pressure of about1-50 bar.

In one or more embodiments, the method further comprises removing theadsorbent from the treated aqueous solution.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a graphical presentation showing the point of zero charge ofthe adsorbent membrane based on the graph of ΔpH (pH₁−pH_(e)) versusinitial pH (pH₁) according to Example 2.

FIG. 2 is an SEM image of the dye-free adsorbent membrane according toExample 2.

FIG. 3 is a graphical presentation of the EDX spectra of the dye-freeadsorbent membrane according to Example 2.

FIGS. 4A and 4B are graphical presentation of the elemental mappingresult of the dye-free adsorbent membrane according to Example 2.

FIG. 5 is an SEM image of the adsorbent membrane adsorbed with MBaccording to Example 2.

FIG. 6 is a graphical presentation of the EDX spectra of the adsorbentmembrane adsorbed with MB according to Example 2.

FIGS. 7A-7D are graphical presentation of the elemental mapping resultof the adsorbent membrane adsorbed with MB according to Example 2.

FIG. 8 is an SEM image of the adsorbent membrane adsorbed with RhBaccording to Example 2.

FIG. 9 is a graphical presentation of the EDX spectra of the adsorbentmembrane adsorbed with RhB according to Example 2.

FIGS. 10A-10C are graphical presentation of the elemental mapping resultof the adsorbent membrane adsorbed with RhB according to Example 2.

FIG. 11 is a graphical presentation of the chemical structure of MBaccording to Example 2.

FIG. 12 is a graphical presentation of the chemical structure of RhBaccording to Example 2.

FIG. 13 is a graphical presentation of the FTIR spectra of the dye-freeadsorbent membrane, the adsorbent membrane adsorbed with MB, and theadsorbent membrane adsorbed with RhB according to Example 2.

FIG. 14 is a graphical presentation of the effect of the initialconcentration of MB on the amount of MB adsorbed per unit mass of theadsorbent membrane according to Example 3.

FIG. 15 is a graphical presentation of the effect of the initialconcentration of RhB on the amount of RhB adsorbed per unit mass of theadsorbent membrane according to Example 3.

FIG. 16 is a graphical presentation of the effect of the adsorbentdosage on the MB removal efficiency of the adsorbent represented by line(a), and of the effect of the adsorbent dosage on the amount of MBadsorbed per unit mass of the adsorbent membrane represented by line(b), with the initial concentration of MB at 50 mg/L, according toExample 4.

FIG. 17 is a graphical presentation of the effect of the adsorbentdosage on the RhB removal efficiency of the adsorbent represented byline (a), and of the effect of the adsorbent dosage on the amount of RhBadsorbed per unit mass of the adsorbent membrane represented by line(b), with the initial concentration of RhB at 50 mg/L, according toExample 4.

FIG. 18 is a graphical presentation of the effect of the adsorbentdosage on the MB removal efficiency of the adsorbent represented by line(a), and of the effect of the adsorbent dosage on the amount of MBadsorbed per unit mass of the adsorbent membrane represented by line(b), with the initial concentration of MB at 100 mg/L, according toExample 4.

FIG. 19 is a graphical presentation of the effect of the adsorbentdosage on the RhB removal efficiency of the adsorbent represented byline (a), and of the effect of the adsorbent dosage on the amount of RhBadsorbed per unit mass of the adsorbent membrane represented by line(b), with the initial concentration of RhB at 100 mg/L, according toExample 4.

FIG. 20 is a graphical presentation of the effect of pH on the MBremoval efficiency of the adsorbent according to Example 5.

FIG. 21 is a graphical presentation of the effect of pH on the RhBremoval efficiency of the adsorbent according to Example 5.

FIG. 22 is a graphical presentation of the Zwitterionic form of RhBaccording to Example 5.

FIG. 23 is a graphical presentation of the MB removal efficiency of theadsorbent at various times of the adsorption process according toExample 6.

FIG. 24 is a graphical presentation of the RhB removal efficiency of theadsorbent at various times of the adsorption process according toExample 6.

FIG. 25 is a graphical presentation of the pseudo-first order kineticmodeling of MB adsorption by the adsorbent according to Example 8.

FIG. 26 is a graphical presentation of the pseudo-first order kineticmodeling of RhB adsorption by the adsorbent according to Example 8.

FIG. 27 is a graphical presentation of the simple first order kineticmodeling of MB adsorption by the adsorbent according to Example 8.

FIG. 28 is a graphical presentation of the simple first order kineticmodeling of RhB adsorption by the adsorbent according to Example 8.

FIG. 29 is a graphical presentation of the pseudo-second order kineticmodeling of MB adsorption by the adsorbent according to Example 8.

FIG. 30 is a graphical presentation of the pseudo-second order kineticmodeling of RhB adsorption by the adsorbent according to Example 8.

FIG. 31 is a graphical presentation of Ritchie's second order kineticmodeling of MB adsorption by the adsorbent according to Example 8.

FIG. 32 is a graphical presentation of Ritchie's second order kineticmodeling of RhB adsorption by the adsorbent according to Example 8.

FIG. 33 is a graphical presentation of the intraparticle diffusionmodeling of MB adsorption by the adsorbent according to Example 8.

FIG. 34 is a graphical presentation of the intraparticle diffusionmodeling of RhB adsorption by the adsorbent according to Example 8.

FIG. 35 is a graphical presentation of the Elovich kinetic modeling ofMB adsorption by the adsorbent according to Example 8.

FIG. 36 is a graphical presentation of the Elovich kinetic modeling ofRhB adsorption by the adsorbent according to Example 8.

FIG. 37 is a graphical presentation of the effect of temperature on theamount of MB adsorbed per unit mass of the adsorbent membrane at varioustimes of the adsorption process according to Example 9.

FIG. 38 is a graphical presentation of the effect of temperature on theamount of RhB adsorbed per unit mass of the adsorbent membrane atvarious times of the adsorption process according to Example 9.

FIG. 39 is a graphical presentation showing the MB and RhB removalefficiencies of the adsorbent following one, two, three, and four cyclesof adsorption and desorption with 0.1M HCl according to Example 10.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present disclosure provides a method of removing at least onecationic dye from an aqueous solution. The method includes contactingthe aqueous solution with an adsorbent comprising a water-insolublemembrane disposed on a substrate. The water-insoluble membrane comprisescross-linked humic acid, at least one alginate, and hydroxyethylcellulose. The contacting forms a treated aqueous solution having alower concentration of the at least one cationic dye relative to theaqueous solution.

In some embodiments, the aqueous solution comprises at least oneselected from the group consisting of industrial waste water, tap water,ground water, river water, and runoff streams.

In some embodiments, the at least one cationic dye is selected from thegroup consisting of methylene blue, rhodamine B, crystal violet, basicfuchsin safranin, pararosaniline, and a combination thereof.

Humic acid is a principal component of humic substances, which are themajor organic constituents of soil (humus), peat, coal, many uplandstreams, dystrophic lakes, and ocean water. It is produced bybiodegradation of dead organic matter. It is not a single acid; rather,it is a complex mixture of many different acids containing carboxyl andphenolate groups so that the mixture behaves functionally as a dibasicacid or, occasionally, as a tribasic acid. Humic acids can formcomplexes with ions that are commonly found in the environment creatinghumic colloids.

Humic acid is dark brown to black in color and is considered to be acomplex aromatic macromolecule with various linkages between thearomatic groups, with a molecular weight of from about 800 daltons toabout 500,000 daltons. The different compounds involved in linkagesinclude amino acids, amino sugars, peptides, aliphatic acids and otheraliphatic compounds. For example, a typical humic acid may have avariety of components including quinone, phenol, catechol and sugarmoieties.

The carboxylic, phenolic, aliphatic and enolic-hydroxyl and carbonyl arethe various functional groups in humic acids. The functional groups thatcontribute most to surface charge and reactivity of humic acids arephenolic and carboxylic groups. Humic acids behave as mixtures ofdibasic acids, with a pK1 value around 4 for protonation of carboxylgroups and around 8 for protonation of phenolate groups. There isconsiderable overall similarity among individual humic acids. The otherimportant characteristic is charge density. The humic acid molecules mayform a supramolecular structure held together by non-covalent forces,such as Van der Waals force, π-π, and CH-π bonds. The presence ofcarboxylate and phenolate groups gives humic acids the ability to formcomplexes with ions such as Mg²⁺, Ca²⁺, Fe²⁺ and Fe³⁺. Many humic acidshave two or more of these groups arranged so as to enable the formationof chelate complexes.

Humic acid is an association of molecules forming aggregates ofelongated bundles of fibers at low pHs and open flexible structuresperforated by voids at high pHs. The voids can trap and adsorb bothorganic and inorganic particles if the charges are complementary.

The sorption of chemicals onto the surfaces of humic substances has beenstudied by a large number of environmental chemists. Sorption mechanismsinclude Van der Waals attractions, hydrophobic bonding, hydrogenbonding, charge transfer, ion exchange, and ligand exchange.

In some embodiments, the humic acid used to form the water-insolublemembrane has a weight average molecular weight of 800-500,000 Da,preferably 1,000-450,000 Da, preferably 2,500-400,000 Da, preferably5,000-350,000 Da, more preferably 7,500-300,000 Da, more preferably10,000-250,000 Da, more preferably 25,000-200,000 Da, more preferably50,000-150,000 Da, or more preferably 75,000-100,000 Da. In otherembodiments, the humic acid used to form the water-insoluble membranehas a weight average molecular weight of at least 1,000 Da, at least5,000 Da, at least 10,000 Da, at least 25,000 Da, at least 50,000 Da,preferably at least 100,000 Da, preferably at least 150,000 Da, morepreferably at least 200,000 Da, more preferably at least 250,000 Da,more preferably at least 300,000 Da, or more preferably at least 400,000Da.

Humic acid dissolves in water at elevated pH under certain conditions,such as in the presence of monovalent species (e.g. alkaline salts andthe like). Humic acid cross-linked by a cross-linking agent has a lowsolubility in water at neutral or higher than neutral pH. Exemplarycross-linking agents include aldehydes and oxidoreductase enzymes,specifically, for example, glutaraldehyde or a mixture of glutaraldehydeand mineral acid (HCl, HNO₃, H₂SO₄, H₃PO₄, etc.).

Among the aldehydes that can be used for cross-linking humic acid arealiphatic or aromatic aldehydes having from 1 to 22 carbon atoms. Thealdehydes may be substituted with any substituent that does notadversely affect the cross-linking capabilities of the aldehydes. Thealdehydes may be saturated or unsaturated. The aldehyde may be anaromatic aldehyde, such as benzaldehyde, tolualdehyde (o-, m-, or p-) orsalicylaldehyde.

Any type of oxidoreductase enzyme can be used to cross-link the humicacid, including peroxidases and hydrogenases.

The cross-linking is effected by reacting the humic acid with thealdehyde or oxidoreductase enzymes such as peroxidase enzymes at roomtemperature or slightly above room temperature for a period of two tofive hours.

Refined from brown seaweeds, alginates are natural anionicpolysaccharides made up by D-mannuronic and L-guluronic acid residuesjoined linearly by 1-4 glycosidic linkages. Alginates from differentspecies of brown seaweed often have variations in their chemicalstructure, resulting in different physical properties. For example, somemay yield an alginate that gives a strong gel, while others may yield aweaker gel. Alginates are commonly available as a sodium or potassiumsalt (i.e., sodium alginate or potassium alginate). Natural alginatesmay be chemically modified to obtain synthetic alginates with improvedbiocompatibility and more desirable physiochemical properties, such asalginate polymer stability, pore size, andhydrophobicity/hydrophilicity.

The viscosity of an alginate solution can vary, depending on thealginate concentration, length of the alginate molecules, or the numberof monomer units in the chains, or the weight average molecular weightof an alginate polymer (the weight average molecular weight of sodiumalginate typically ranges from 10,000 to 600,000 Da), with longer chainsresulting in higher viscosities at similar concentrations. For example,a low viscosity sodium alginate available from Sigma Aldrich has aviscosity of 4-12 cP when dissolved in water at a concentration of 1% at25° C. A medium viscosity sodium alginate available from Sigma Aldrichhas a viscosity of no less than 2,000 cP when dissolved in water at aconcentration of 2% at 25° C. A high viscosity sodium alginate availablefrom Sigma Aldrich has a viscosity of about 14,000 cP when dissolved inwater at a concentration of 2% at 25° C.

An alginate can be cross-linked by one or more of the above mentionedcross-linking agents, preferably a mixture of glutaraldehyde and mineralacid (HCl, HNO₃, H₂SO₄, H₃PO₄, etc.), to form a water-insolublealginate. When combined with cross-linked humic acic, a cross-linkedalginate and/or a cross-linked blend of alginate and humic acid furtherlower the solubility of the cross-linked humic acid, thus providing asolid support to further immobilize the cross-linked humic acid.

In one embodiment, the at least one alginate used to form thewater-insoluble membrane of the adsorbent comprises a low viscosityalginate or alginate salt. In another embodiment, the at least onealginate used to form the water-insoluble membrane of the adsorbentcomprises a medium viscosity alginate or alginate salt. In still anotherembodiment, the at least one alginate used to form the water-insolublemembrane of the adsorbent comprises a high viscosity alginate oralginate salt. The at least one alginate used to form thewater-insoluble membrane of the adsorbent can be ammonia alginate,sodium, potassium, magnesium or calcium alginate.

In some embodiments, the at least one alginate used to form thewater-insoluble membrane of the adsorbent has a weight average molecularweight of 10,000-600,000 Da, or preferably 25,000-500,000 Da, orpreferably 40,000-400,000 Da, or preferably 55,000-300,000 Da, orpreferably 70,000-200,000 Da, or preferably 85,000-100,000 Da. In otherembodiments, the at least one alginate used to form the water-insolublemembrane of the adsorbent has a weight average molecular weight of atleast 20,000 Da, at least 50,000 Da, preferably at least 100,000 Da,more preferably at least 250,000 Da, more preferably at least 350,000Da, or more preferably at least 400,000 Da, or more preferably at least500,000 Da.

In some embodiments, the at least one alginate used to form thewater-insoluble membrane of the adsorbent comprises one or moreunmodified, or natural alginates.

Natural alginates may be modified to obtain synthetic alginates withimproved biocompatibility and more desirable physiochemical properties,such as alginate polymer stability, pore size, andhydrophobicity/hydrophilicity. In other embodiments, the at least onealginate used to form the water-insoluble membrane of the adsorbentcomprises one or more modified, or synthetic, alginates, such as thosedisclosed in U.S. Patent Application US20120308650 A1, incorporatedherein by reference in its entirety. One embodiment of the modifiedalginate comprises one or more covalently modified monomers defined byFormula I,

wherein, X is oxygen, sulfur, or NR; R₁ is hydrogen, or an organicgrouping containing any number of carbon atoms, preferably 1-30 carbonatoms, more preferably 1-20 carbon atoms, more preferably 1-14 carbonatoms, more preferably 2-10 carbon atoms, more preferably 3-8 carbonatoms, more preferably 4-6 carbon atoms, and optionally including one ormore heteroatoms such as oxygen, sulfur, or nitrogen grouping in linear,branched, or cyclic structural formats, representative R₁ groupingsbeing alkyl, substituted alkyl, alkenyl, substituted alkenyl, alkynyl,substituted alkynyl, phenyl, substituted phenyl, aryl, substituted aryl,heteroaryl, substituted heteroaryl, alkoxy, substituted alkoxy, phenoxy,substituted phenoxy, aroxy, substituted aroxy, alkylthio, substitutedalkylthio, phenylthio, substituted phenylthio, arylthio, substitutedarylthio, carbonyl, substituted carbonyl, carboxyl, substitutedcarboxyl, amino, substituted amino, amido, substituted amido, sulfonyl,substituted sulfonyl, sulfonic acid, phosphoryl, substituted phosphoryl,phosphonyl, substituted phosphonyl, polyaryl, substituted polyaryl,C₃-C₂₀ cyclic, substituted C₃-C₂₀ cyclic, heterocyclic, substitutedheterocyclic, aminoacid, poly(ethylene glycol), peptide, or polypeptidegroup; Y₁ and Y₂ independently are hydrogen or —PO(OR)₂; or Y₂ isabsent, and Y₂, together with the two oxygen atoms to which Y₁ and Y₂are attached form a cyclic structure as shown below,

wherein n is an integer between 1 and 4; and R₂ and R₃ are,independently, hydrogen or an organic grouping containing any number ofcarbon atoms, preferably 1-30 carbon atoms, more preferably 1-20 carbonatoms, more preferably 1-14 carbon atoms, and optionally including oneor more heteroatoms such as oxygen, sulfur, or nitrogen grouping inlinear, branched, or cyclic structural formats, representative Rgroupings being alkyl, substituted alkyl, alkenyl, substituted alkenyl,alkynyl, substituted alkynyl, phenyl, substituted phenyl, aryl,substituted aryl, heteroaryl, substituted heteroaryl, alkoxy,substituted alkoxy, phenoxy, substituted phenoxy, aroxy, substitutedaroxy, alkylthio, substituted alkylthio, phenylthio, substitutedphenylthio, arylthio, substituted arylthio, carbonyl, substitutedcarbonyl, carboxyl, substituted carboxyl, amino, substituted amino,amido, substituted amido, polyaryl, substituted polyaryl, C₃-C₂₀ cyclic,substituted C₃-C₂₀ cyclic, heterocyclic, substituted heterocyclic,aminoacid, poly(ethylene glycol), peptide, or polypeptide group; or R₂and R₃, together with the carbon atom to which they are attached, form a3- to 8-membered unsubstituted or substituted carbocyclic orheterocyclic ring; and R is, independently for each occurrence, hydrogenor an organic grouping containing any number of carbon atoms, preferably1-30 carbon atoms, more preferably 1-20 carbon atoms, more preferably1-14 carbon atoms, and optionally including one or more heteroatoms suchas oxygen, sulfur, or nitrogen grouping in linear, branched, or cyclicstructural formats, representative R groupings being alkyl, substitutedalkyl, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl,phenyl, substituted phenyl, aryl, substituted aryl, heteroaryl,substituted heteroaryl, alkoxy, substituted alkoxy, phenoxy, substitutedphenoxy, aroxy, substituted aroxy, alkylthio, substituted alkylthio,phenylthio, substituted phenylthio, arylthio, substituted arylthio,carbonyl, substituted carbonyl, carboxyl, substituted carboxyl, amino,substituted amino, amido, substituted amido, polyaryl, substitutedpolyaryl, C₃-C₂₀ cyclic, substituted C₃-C₂₀ cyclic, heterocyclic,substituted heterocyclic, aminoacid, poly(ethylene glycol), peptide, orpolypeptide group.

In other embodiments, the at least one alginate used to form thewater-insoluble membrane of the adsorbent may comprise a combination ofat least one unmodified alginate and at least one modified alginate. Insome embodiments, the mass ratio of the modified alginate(s) to theunmodified alginate(s) ranges from 15:1 to 1:15, 10:1 to 1:10, 5:1 to1:5. 2:1 to 1:2, or 1:1.

Because the cross-linked alginates are a porous matrix/gel, when theimmobilized cross-linked humic acid is contacted with the aqueoussolution containing the cationic dyes, the aqueous solution can diffusethrough the cross-linked alginates to contact the cross-linked humicacid.

In some embodiments, the cross-linked at least one alginate forming thewater-insoluble membrane may be substituted by or mixed withcross-linked dextran gels, agar, gellan, chitosan, and curdlan. In otherembodiments, the water-insoluble membrane may comprise cross-linkedalginates and one or more of cross-linked dextran gels, agar, gellan,chitosan, and curdlan.

Hydroxyethyl cellulose is a gelling and thickening agent derived fromcellulose, and can be cross-linked by one or more of the above mentionedcross-linking agents, preferably a mixture of glutaraldehyde and mineralacid (HCl, HNO₃, H₂SO₄, H₃PO₄, etc.), to form a water-insolublehydroxyethyl cellulose, or a cross-linked blend of hydroxyethylcellulose, at least one alginate, and humic acid when the at least onealginate and humic acid are also present during the cross-linkingreaction according to the present disclosure. Without wishing to bebound by theory, it is believed that the cross-linked hydroxyethylcellulose serves as a water-insoluble binder in the inventive adsorbent,and also improves the cationic dye removal efficiency of the adsorbentby enhancing the contact of the cross-linked humic acid with the aqueoussolution due to the hydrophilic nature of the cross-linked hydroxyethylcellulose and/or by further immobilizing the cross-linked humic acid.The cross-linked hydroxyethyl cellulose itself may have cationic dyeadsorptive activity, and may work synergistically with the cross-linkedhumic acid to achieve a surprisingly high cationic dye removalefficiency of the adsorbent. It is contemplated that the hydroxyethylcellulose used to form the adsorbent membrane may be modified to improvethe cationic dye adsorption/removal efficiency even further.

In some embodiments, the hydroxyethyl cellulose used to form thewater-insoluble membrane has a weight average molecular weight of 10,000to 500,000 Da, preferably 25,000 to 400,000 Da, preferably 50,000 to300,000 Da, preferably 75,000 to 250,000 Da, or preferably 100,000 to200,000 Da. In other embodiments, the hydroxyethyl cellulose used toform the water-insoluble membrane has a weight average molecular weightof at least 10,000 Da, at least 25,000 Da, preferably at least 50,000Da, preferably at least 100,000 Da, more preferably at least 150,000 Da,more preferably at least 200,000 Da, more preferably at least 250,000Da, more preferably at least 300,000 Da, or more preferably at least350,000 Da.

In some embodiments, the adsorbent may further comprise at least onewater-insoluble binder selected from the group consisting of methylcellulose, polyvinyl acetate, polyvinyl chloride, polystyrene andstyrene-butadiene copolymers. The water-insoluble binder facilitates theformation of the membrane and strengthens the membrane formed on thesubstrate surface without affecting the adsorptive characteristics ofthe inventive adsorbent.

In the disclosed adsorbent, the adsorption activity comes from thewater-insoluble membrane obtained by cross-linking humic acid, at leastone alginate, and hydroxyethyl cellulose with a cross-linking agent,e.g. glutaraldehyde, preferably together with a mineral acid as acatalyzer, e.g. HCl, to form a cross-linked blend film or membrane,whereas the substrate does not have significant adsorption activity butprovides a support for the membrane. In the cross-linked blend membrane,the components of humic acid involved in the cross-linkages may includeamino acids, amino sugars, peptides, aliphatic acids, aliphatic and/oraromatic alcohols, and other aliphatic compounds. The components of theat least one alginate involved in the cross-linkages may include thecarboxylic and aliphatic and enolic-hydroxyl groups. The components ofhydroxyethyl cellulose involved in the cross-linkages may include one ormore of the hydroxyethyl alcohol groups. All the above mentionedcross-linked components may cross-link among the same or one another viathe cross-linking agent to form a blended statistical polymer. Forexample, the humic acid may be cross-linked to another humic acid,alginate, hydroxyethyl cellulose, or mixtures thereof. The weight ratioof humic acid:at least one alginate:hydroxyethyl cellulose in themembrane-forming solution (described below) may dictate the weightpercentage of each substance in the cross-linked polymer. The weightpercentages of the membrane-forming substances may be altered to alterthe properties of the membrane, including the properties of cationic dyeadsorption, such as adsorption capacity, adsorption efficiency, andadsorption kinetics, etc.

In some embodiments, in the water-insoluble membrane, humic acid ispresent in an amount of about 5-30%, preferably about 8-25%, preferablyabout 10-20%, or more preferably about 12-15%, of the total weight ofthe water-insoluble membrane; the at least one alginate is present in anamount of about 40-90%, preferably about 50-84%, preferably about60-80%, or more preferably about 70-76%, of the total weight of thewater-insoluble membrane; and hydroxyethyl cellulose is present in anamount of about 5-30%, preferably about 8-25%, preferably about 10-20%,or more preferably about 12-15%, of the total weight of thewater-insoluble membrane. In other embodiments, in the water-insolublemembrane, the weight ratio of humic acid:at least onealginate:hydroxyethyl cellulose lies in the range (5-30):(40-90):(5-30),or preferably (8-25):(50-84):(8-25), or preferably(10-20):(60-80):(10-20), or more preferably (12-15):(70-76):(12-15).

The substrate of the adsorbent can comprise any suitable material whichis inert or stable in water or an aqueous solution during a cationic dyeadsorption process and which provides a surface for the disposition orattachment of the water-insoluble membrane. Non-limiting examples of thematerial the substrate may comprise include one or more of such polymersas polypropylene, polystyrene, PET (polyethylene terephthalate),polyimide, PEN (polyethylene naphthalate), agarose, acetate cellulose,and PC (polycarbonate), and other polymers, glass, plastic, rubber, ametal, an alloy, a ceramic, a carbonaceous material, sand, silicon, andsilica.

The thickness and shape of the substrate can vary. Preferably, thesubstrate is just thick enough to provide sufficient support for themembrane and maintain the integrity of the adsorbent, e.g. the adsorbentdoes not break or disintegrate during the contacting with the aqueoussolution to adsorb the cationic dyes, during the removal of theadsorbent from the aqueous solution when the adsorption is complete,and/or during the regeneration of the adsorbent, without addingunnecessary weight and/or volume to the adsorbent. Non-limiting examplesof the shapes of the substrate include a granule, a pellet, a sphere, apowder, a woven fabric, a non-woven fabric, a mat, a felt, a block, anda honeycomb.

There are different ways of disposing the water-insoluble membrane onthe substrate. For example, the humic acid, the at least one alginate,and the hydroxyethyl cellulose may first be dissolved in water or anaqueous solution to form a membrane-forming solution, and then thesubstrate may be immersed or dipped in the membrane-forming solution, orthe membrane-forming solution may be sprayed onto the substrate, or themembrane-forming solution may be applied onto the surface of thesubstrate by brushing, etc, such that the membrane-forming solutionforms a coating on the substrate. The substrate with themembrane-forming solution coating can be air dried or dried at anelevated temperature, e.g. in an heated oven, such that the wet coatingof the membrane-forming solution turns into a dry membrane comprisingthe humic acid, the at least one alginate, and the hydroxyethylcellulose, which are then cross-linked with one or more of thecross-linking agents described above to form the water-insolublemembrane conferring the adsorption activity of the adsorbent for thecationic dyes. In some embodiments, the above mentioned process ofdepositing and cross-linking the membrane-forming substances of humicacid, at least one alginate, and hydroxyethyl cellulose may be repeatedone or more times to increase the coverage and/or thickness of themembrane on the substrate.

In some embodiments, in order to strengthen the attachment of thewater-insoluble membrane to the substrate, one or more membrane-formingsubstances may comprise or may be modified to comprise a first molecularmoiety, e.g. the alginate may be covalently conjugated with biotin, andthe substrate may comprise or may be modified to comprise a secondmolecular moiety capable of binding to the first molecular moiety,preferably with high affinity. For example, the substrate may comprise apolymer, such as agarose, conjugated with avidin or streptavidin, or thesubstrate may comprise a plastic, such as polystyrene, of which surfaceis coated with avidin or streptavidin. Both avidin and streptavidin bindto biotin with high affinity. Conjugation of alginate with biotin isdescribed by Polyak Bl, Geresh S, Marks R S, Synthesis andcharacterization of a biotin-alginate conjugate and its application in abiosensor construction, Biomacromolecules. 2004 March-April;5(2):389-96, incorporated herein by reference in its entirety.Conjugation of polymers, e.g. agarose, with avidin or streptavidin iswell known in the art. Coating of a plastic surface with avidin orstreptavidin is also well known in the art. As another example, thealginate may be covalently conjugated with heparin, as disclosed by ZuoQl, Guo R, Liu Q, Hong A, Shi Y, Kong Q, Huang Y, He L, Xue W.,Heparin-conjugated alginate multilayered microspheres for controlledrelease of bFGF., Biomed Mater. 2015 Jun. 4; 10(3):035008, incorporatedherein by reference in its entirety. The substrate may be a plastic,such as polystyrene, of which surface may be coated with a protein orpeptide that binds to heparin with high affinity. Heparin bindingproteins are well known in the art, including, without limitation,fibroblast growth factor, azurocidin, and pleiotrophin. The peptide maybe selected based on the regions of the heparin binding proteins thatconfer the heparin-binding activity. The techniques of coating of aplastic surface with a protein or a peptide are also well known in theart, one of which is disclosed by Cuccuru MA1, Dessi D, Rappelli P,Fiori PL, A simple, rapid and inexpensive technique to bind smallpeptides to polystyrene surfaces for immunoenzymatic assays, J ImmunolMethods. 2012 Aug. 31; 382(1-2):216-9, incorporated herein by referencein its entirety. As still another example, the alginate may becovalently conjugated with a laminin peptide that binds to an integrin,as disclosed by Yamada Yl, Hozumi K, Katagiri F, Kikkawa Y, Nomizu M,Biological activity of laminin peptide-conjugated alginate and chitosanmatrices, Biopolymers. 2010; 94(6):711-20, incorporated herein byreference in its entirety. The substrate may be again a plastic whosesurface is coated with the integrin protein or a peptide derived fromthe integrin protein that confers the binding activity. As still anotherexample, the substrate may be hollow glass microspheres coated withnanoparticles of Fe₃O₄, as disclosed in Chinese Patent No. CN103043916B, incorporated herein by reference in its entirety. The nanoparticlesof Fe₃O₄ confer the hollow glass beads with magnetic properties.Additionally, the nanoparticles of Fe₃O₄ coating the hollow glassmicrosphere substrate can bind to the humic acid of the water-insolublemembrane through coordination, as disclosed in Chinese Patent No.CN103752281 A, incorporated herein by reference in its entirety.

In some embodiments, the water-insoluble membrane covers at least 50%,preferably at least 60%, more preferably at least 70%, more preferablyat least 80%, more preferably at least 90%, or more preferably at least95%, of the surface of the substrate.

The thickness of the water-insoluble membrane disposed on the substratemay vary, depending, for example, on the concentrations of the humicacid, the at least one alginate, and the hydroxyethyl cellulose in themembrane-forming solution coating or disposing on the substrate, theduration of the coating or disposition, and/or the number of times ofrepeated coating or disposition. In some embodiments, the thickness ofthe water insoluble membrane is at least 50 μm, preferably at least 150more preferably at least 250 μm, more preferably at least 350 μm, morepreferably at least 500 μm, more preferably at least 750 μm, morepreferably at least 900 μm, or more preferably at least 1 mm. A suitablythick membrane provides sufficient adsorption capacity or sites andmaintains integrity and stability under various adsorption conditions,including, but not limited to, the temperature of the adsorptionprocess, the concentrations of the cationic dyes and the nature andconcentrations of other compositions of the aqueous solution that canaffect the integrity and stability of the adsorbent membrane, and thedesired number of regeneration and reuse cycles of the adsorbentmembrane.

In some embodiments, the pH of the aqueous solution is about 1-12, about3-10, about 4-8, or about 5-7. In some embodiments, when the at leastone cationic dye comprises methylene blue, the pH of the aqueoussolution is preferably 6-10, or more preferably 7-9. In otherembodiments, when the at least one cationic dye comprises rhodamine B,the pH of the aqueous solution is preferably 1 to 7, or more preferably2-6, or more preferably 3-5.

In some embodiments, the at least one cationic dye is methylene blue,and the adsorbent removes at least 50%, preferably at least 60%,preferably at least 70%, more preferably at least 80%, or morepreferably at least 90%, or more preferably at least 95%, of themethylene blue from the aqueous solution.

In some embodiments, the at least one cationic dye is rhodamine B, andthe adsorbent removes at least 50%, preferably at least 60%, preferablyat least 70%, more preferably at least 80%, or more preferably at least90%, or more preferably at least 95%, of the rhodamine B from theaqueous solution

The initial concentration of the rhodamine B in the aqueous solutionaffects the efficiency of removing rhodamine B from the aqueous solutionwith the adsorbent. The initial concentration of the rhodamine B in theaqueous solution is preferably lower than 75 mg/L, preferably lower than60 mg/L, or more preferably lower than 50 mg/L.

The water-insoluble membrane of the adsorbent confers the dye adsorptionactivity. The concentration of the water-insoluble membrane contactingthe aqueous solution containing the cationic dyes can vary, depending onthe initial concentration of the cationic dyes in the aqueous solution,which may range from about less than 10 mg/L to 100 mg/L, the removalefficiency required, the availability of the adsorbent, the capacity forthe treatment of the aqueous solution with the adsorbent, e.g. the sizeof a vessel used for batch adsorption, and the capacity for filtrationof the adsorbent from the aqueous solution to remove and/or regeneratethe adsorbent, etc. A typical concentration of the water-insolublemembrane treating the aqueous solution is about 0.04-0.64 g/100 mL ofthe aqueous solution, about 0.08-0.50 g/100 mL of the aqueous solution,or about 0.10-0.40 g/100 mL of the aqueous solution, or about 0.15-0.30g/100 mL of the aqueous solution, or about 0.20-0.25 g/100 mL of theaqueous solution.

Besides batch adsorption, e.g. by letting the aqueous solution remain incontact with the adsorbent for a time sufficient to remove the cationicdyes, granular particles of the adsorbent may be installed in a fixedbed reactor or fluidized bed reactor. For example, the aqueous solutioncontaining the cationic dyes can be applied to a fixed bed column of theadsorbent, and the effluent of the column comprises the treated aqueoussolution with reduced concentrations of the cationic dyes. In someembodiments, the fixed bed reactor of the adsorbents comprises acartridge for easy carry and use. For example, such a cartridge can beattached to a faucet of tap water or ground water, or installed in acontainer where the aqueous solution passes through the cartridge froman upper level of the container, with the treated aqueous solutionexiting the cartridge at a lower level of the container with reducedconcentrations of the cationic dyes. Further, the cartridge can includeother cationic dye adsorbents such as activated carbon.

Alternatively, the adsorbent can form a fluidized bed reactor with theaqueous solution containing the cationic dyes, for example, byintroducing the pressurized aqueous solution through the particulatemedium of the adsorbent. In the fluidized bed reactor, contact betweenthe adsorbent and the aqueous solution is greatly enhanced as comparedto a fixed bed column or reactor, leading to a higher removal efficiencyof the cationic dyes from the aqueous solution.

Additionally, the adsorbent can take a variety of forms that may be ormay not be shaped by the forms of the substrate to facilitate removal ofthe cationic dyes from the aqueous solution and/or removal of theadsorbent from the aqueous solution when the adsorption is complete,and/or when the adsorption capacity of the adsorbent is exhausted andthe adsorbent needs to be regenerated. Non-limiting examples of theforms include a granule, a pellet, a sphere, a powder, a woven fabric, anon-woven fabric, a mat, a felt, a block, and a honeycomb.

In some embodiments, the method further comprises removing the adsorbentfrom the treated aqueous solution. For example, the adsorbent in powderform may be injected into an aqueous solution storage tank and thenremoved by filtration, centrifugation, or settling. The adsorbent infiber form may be inserted in a section of the aqueous solution or wastewater treatment piping or trench, and optionally be removed forregeneration when its adsorption capacity has been exhausted andreplaced by fresh adsorbent. In some embodiments, the adsorbent mayfurther comprise a magnetic material, for example, the adsorbent may bein the form of a magnetic sphere or bead, such that the adsorbent can beeasily removed from the treated aqueous solution with a magnet.

In one embodiment, the method further comprises regenerating theadsorption ability of the adsorbent. In some embodiments, theregenerating the adsorption ability of the adsorbent comprises treatingthe adsorbent with at least one mineral or strong acid, e.g. a HClsolution at a concentration of 0.01-1 M, preferably 0.05-0.8 M,preferably 0.08-0.5 M, preferably 0.1-0.3 M, by, for example and withoutlimitation, soaking the adsorbent in the HCl solution with agitation orultra-sonication for a period of time, e.g. about 2-10 hours, about 3-8hours, or about 4-6 hours, to effectively desorb the cationic dyes fromthe adsorbent. One or more other mineral or strong acids and/or organicacids that can be used in place of HCl or in combination with HCl toregenerate the adsorption ability of the adsorbent include, withoutlimitation, nitric acid, hydrobromic acid, hydroiodic acid, formic acid,hydrofluoric acid, sulfuric acid, and chloric acid, acetic acid,propionic acid, butyric acid, valeric acid, caproic acid, oxalic acid,lactic acid, malic acid, citric acid, benzoic acid, carbonic acid, andthe like. After the acid treatment, in some embodiments, the membrane ofthe adsorbent may be thoroughly washed with distilled water by soakingor rinsing till the pH of the water from the washing is neutral. In someembodiments, the adsorbent, particularly the water-insoluble membrane,is dried and reused. In some embodiments, the adsorbent undergoes 1-10cycles, or 2-8 cycles, or 3-7 cycles, or 4-6 cycles, ofregeneration-reuse without a significant loss of the adsorption capacityfor the cationic dyes.

In another embodiment, the method of using the adsorbent to remove thecationic dyes from the aqueous solution may take a form of continuousand/or multi-stage adsorption with the adsorbent. For example, multiplefixed bed columns or reactors of the adsorbent or, more broadly,multiple adsorption units of any suitable modes or configurations andtheir combinations, e.g. batch adsorption, cartridge, fluidized bedreactor, etc., can be set up to remove the cationic dyes from theaqueous solution in a parallel and/or sequential manner. In someembodiments, the adsorption columns, reactors, or units set up in theparallel fashion may be standby columns, reactors, or units ready toreplace another set of parallel columns, reactors, or units whoseadsorption capacity has been exhausted to make the removal operationcontinuous. The adsorbent in replaced columns, reactors, or units may beregenerated and reused. In other embodiments, the adsorption columns,reactors, or units set up in the sequential or serial fashion allow thecationic dyes to be removed from the aqueous solution through multiplestages to achieve a high removal efficiency.

In some embodiments, the aqueous solution is contacted with theadsorbent at a temperature of about 4-100° C., preferably about 10-90°C., preferably about 20-80° C., preferably about 30-70° C., orpreferably about 40-60° C.

In some embodiments, the aqueous solution is contacted with theadsorbent at a pressure of about 1-100 bar, about 1-80 bar, preferablyabout 1-50 bar, preferably about 1-30 bar, preferably about 1-20 bar, orpreferably about 1-10 bar.

Having generally described this invention, a further understanding canbe obtained by reference to certain specific examples which are providedherein for purposes of illustration only and are not intended to belimiting unless otherwise specified.

Example 1 Materials and Methods 1. Materials

2-hydroxy ethyl cellulose (HEC) (M_(w)˜250,000 Da), methylene blue (MB)(M_(w)˜373.9), and rhodamine B (RhB) (M_(w)-479.01) were procured formSigma Aldrich Co. Sodium alginate (SA) and sodium salt of humic acid(HA) used for the preparation of the water-insoluble membrane of theadsorbent were obtained from Himedia, Mumbai. Glutaraldehyde (GA) (25%solution) was obtained from Merck India Ltd. NaOH and HCl required forpH studies were of analytical grade. All the chemicals were used withoutfurther purification.

2. Methods 2.1. Preparation of the Adsorbent

0.4 g of HA, 2.2 g of SA, and 0.4 g of HEC were dissolved in 100 ml ofdistilled water for over 24 h to obtain a membrane-forming solution,with the combined weight of the membrane-forming substances HA, SA, andHEC being 3% of the total weight of the membrane-forming solution. Afterthe dissolution, the membrane-forming solution was subjected tocentrifugation to remove any undissolved particles. The membrane-formingsolution was then poured onto a polypropylene substrate pasted on aglass plate. The polypropylene substrate provided a strong support forthe adsorbent membrane and made the handling of the adsorbent membraneconvenient and easy. The polypropylene substrate of which surface wascoated with the membrane-forming solution was then dried in an oven toform a dried membrane coating the surface of the polypropylenesubstrate. To cross-link the dried membrane comprising the HA, the SA,and the HEC, the polypropylene substrate with the dried membrane coatingwas immersed in an aqueous solution containing 70% methanol, 2.5 wt %HCl and 2.5 wt % GA for 4 h to form the adsorbent. The cross-linkedwater-insoluble membrane of the adsorbent was then thoroughly washedwith distilled water before being subjected to characterization and dyeremoval studies.

2.2. Determination of the Point of Zero Charge of the Water-InsolubleMembrane of the Adsorbent

Zero point charge (P_(zc)) of the adsorbent membrane describes thecondition at which the surface of the adsorbent membrane has zeroelectrical charge density. P_(zc) was determined by a conventionalmethod. Specifically, 25 ml of 0.1 M NaCl solution was placed in each ofa series of 100 ml Erlenmeyer flasks, with the pH of the 0.1 M NaClsolution in each flask adjusted to a value between 1 and 9 by theaddition of a 0.1 M HCl solution or a 0.1 M NaOH solution. The pH of thesolution was measured on EQUIP-TRONIC (Model EQ-610). The initial pH ofthe 0.1 M NaCl solution was designated as pH₁. 0.1 g of the adsorbentcomprising the adsorbent membrane disposed on the polypropylenesubstrate was added to each of the flasks containing the pH adjusted 0.1M NaCl solutions. The flasks were placed on an orbital shaker (ScigenicBiotech) at room temperature and shaken at 150 rpm till no difference inthe pH values was observed between two successive readings. Theequilibrium pH was designated as pH_(c) which was used to calculate ΔpH(pH₁−pH_(e)). A graph of ΔpH against pH₁ gave the P_(zc) value at whichthe ΔpH is equal to zero.

2.3. Membrane Characterization

The surface of the adsorbent membrane was investigated by ScanningElectron Microscopy (SEM). In order to confirm the adsorption of dye onthe membrane, elemental mapping and energy dispersive X-ray (EDX)analysis were carried out. For this study, the adsorbent membrane wasinitially deposited with gold by sputtering for increased conductivity.Fourier transform infrared spectra (FTIR) of the adsorbent membranebefore and after the adsorption of the cationic dyes were recorded on aPerkin Elmer Spectrum 100 apparatus.

2.4. Dye Removal Studies

The cationic dye removal efficiency and the amount of dye adsorbed orremoved per unit mass of the adsorbent membrane of the adsorbent wereinvestigated by varying the initial dye concentration, adsorbent dosage,pH and adsorption time. All the experiments were carried out using batchadsorption on an orbital shaker at 27° C. and at a shaking speed of 150rpm. For pH studies and adsorption kinetic studies, the concentration ofthe dye was fixed at 50 mg/L. The concentration of the dye before andafter adsorption was measured using ultraviolet-visible (UV-Vis)spectrophotometer (Analytikjena Specord S600). The concentrations of MBand RhB were measured at the wavelength of 665 nm and 554.5 nm,respectively. From the initial and final concentrations, the dye removalefficiency (%) was calculated according to Equation (1):

$\begin{matrix}{{{Dye}\mspace{14mu} {Removal}\mspace{14mu} (\%)} = {\left( {1 - \frac{C_{e}}{C_{0}}} \right) \times 100}} & (1)\end{matrix}$

Where C_(e) and C₀ are the equilibrium and initial dye concentrations inmg/L, respectively. The amount of dye adsorbed on the adsorbent membranewas calculated according to Equation (2)

$\begin{matrix}{q_{t} = \frac{\left( {C_{0} - C_{t}} \right)V}{m}} & (2)\end{matrix}$

Where q_(t) is the amount of dye adsorbed per unit mass of the adsorbentmembrane (mg/g), m is the mass of the adsorbent membrane (g), V is thevolume of the dye solution (L). As mentioned above, C₀ (mg/L) is theinitial dye concentration and C_(t) (mg/L) is the dye concentration attime t.

2.5. Adsorbent Membrane Regeneration and Reuse

The regeneration and reusability of the adsorbent membrane was studiedby subjecting the adsorbent membrane to multipleadsorption-desorption-adsorption cycles. For desorption, the membraneadsorbed with the dye was added to a 0.1 M HCl solution placed in a 100ml Erlenmeyer flask. The flask was agitated on an orbital shaker at 150rpm for 5 h. Afterwards, the membrane was thoroughly washed withdistilled water till the pH of the distilled water collected from thewashing was neutral. The membrane was dried and then used again for dyeadsorption. The adsorbent membrane was subjected to theadsorption-desorption cycle repeatedly for four times to determine itsreusability.

2.6. Determination of the Weight of the Adsorbent Membrane

In the following examples, a desired amount of the adsorbent was cutinto small pieces, with each piece having an area of approximately 0.5cm². Cutting of the adsorbent into small pieces ensured a large surfacearea of the membrane available for dye adsorption.

The adsorbent comprises the water-insoluble HA/SA/HEC membrane and thepolypropylene substrate support on which the membrane is disposed.Experiments performed with the polypropylene substrate support aloneindicated that it did not play any major role in the dye adsorptionprocess. In order to study the effect of the adsorbent dosage on the dyeadsorption, the weight of the water-insoluble HA/SA/HEC membrane was thedetermining factor. To determine the weight of the water-insolubleHA/SA/HEC membrane in a specific amount of the adsorbent, thepolypropylene substrate was weighed initially, with the weightdesignated as W1. After the coating of the polypropylene substrate withthe HA/SA/HEC, the cross-linking of the HA/SA/HEC to form thewater-insoluble HA/SA/HEC membrane of the adsorbent, and the drying ofthe adsorbent, the adsorbent was weighed, with the weight designated asW2. The difference between W2 and W1 was the weight of thewater-insoluble HA/SA/HEC membrane that conferred the adsorptionactivity. Hence, when the total weight of the adsorbent was 0.1 g, theweight of the water-insoluble HA/SA/HEC membrane was 0.02 g. Likewise,when the total weight of the adsorbent was 0.2 g, the weight of thewater-insoluble HA/SA/HEC membrane was 0.05 g. When the total weight ofthe adsorbent was 0.3 g, the weight of the water-insoluble HA/SA/HECmembrane was 0.08 g. The determination of the weight of thewater-insoluble HA/SA/HEC membrane in each of the above amounts of theadsorbent was performed in triplicates to minimize error.

In the following examples, the amount of the dye adsorbed per unit massof the adsorbent was calculated based on the unit mass of thewater-insoluble HA/SA/HEC membrane of the adsorbent.

Example 2 Characterization of the Adsorbent Membrane 1. Point of ZeroCharge of the Adsorbent Membrane

Referring to FIG. 1, the point of zero charge (P_(zc)) of the adsorbentmembrane was 3.50. The acidic nature of the surface may be due to thepresence of carboxyl and phenolic groups on HA and acid groups on SA.The membrane hence was positively charged at a pH below 3.50, and wasnegative charged at a pH above 3.50, indicating that at a pH above P_(a)adsorption of cations is favored, whereas adsorption of anions isfavored at a pH below P_(Zc).

2. SEM and FTIR Analysis of the Adsorbent Membrane

Referring to FIG. 2, the surface of the adsorbent membrane without anydye adsorbed appeared to be relatively smooth in comparison to thesurface of the adsorbent membrane adsorbed with MB shown in FIG. 5, andin comparison to the surface of the adsorbent membrane adsorbed with RhBshown in FIG. 8. The surface of the membrane adsorbed with the dyeappeared rough, probably due to the dye aggregates formed on themembrane surface.

Dye adsorption was further confirmed by elemental mapping and EDXanalysis. Comparing FIG. 6 and FIGS. 7A, 7B, 7C, and 7D, which are theEDX spectra and elemental mapping result of the membrane adsorbed withMB, respectively, with FIG. 3 and FIGS. 4A and 4B, which are the EDXspectra and elemental mapping result of the dye-free membrane,respectively, the presence of the nitrogen and sulfur peaks in the EDXspectra confirmed that MB had been adsorbed on the membrane, since MBcontains both nitrogen and sulfur as shown in FIG. 11 depicting thechemical structure of MB. Referring to FIG. 9 and FIGS. 10A, 10B, and10C, which are the EDX spectra and elemental mapping result of themembrane adsorbed with RhB, respectively, the presence of the nitrogenpeak in the EDX spectra confirmed that RhB had been adsorbed on themembrane, since RhB contains nitrogen as shown in FIG. 12 depicting thechemical structure of RhB.

Referring to FIG. 13, the FTIR spectra of the dye-free adsorbentmembrane displayed characteristic peaks at 3367 cm⁻¹ corresponding tothe OH stretching frequency of the carboxyl and phenol groups, at 1586cm⁻¹ corresponding to the C═O stretching frequency of the conjugatedcarbonyl group, at 1701 cm⁻¹ corresponding to the C═O stretchingfrequency of the carbonyl group in SA, and at 1115 cm⁻¹ corresponding tothe C═O stretching frequency. When MB or RhB was adsorbed on themembrane, the spectra of the membrane exhibited a decrease in the peakintensity at 3367 cm⁻¹ corresponding to the OH stretching frequency,indicating an interaction between the carboxylate ions of the adsorbentwith the cationic dye. The rest of the spectra of the dye-adsorbedmembranes remained mostly unchanged compared with that of the dye-freemembrane, indicating electrostatic interactions and a lack of anychemical bond formation between the adsorbent membrane and the dyes. Theconcentration of the dyes on the membrane surface was too low to giverise to any noticeable peaks in the IR spectra of the dye adsorbedmembrane.

Example 3

Effect of the initial dye concentration on the amount of dye adsorbed orremoved per unit mass of the adsorbent membrane

Solutions containing MB or RhB at six different initial concentrations,i.e. 5, 15, 30, 50, 75, and 100 mg/L, were prepared with a pH of 7, and0.3 g of the adsorbent was added to each solution.

The initial dye concentration plays a major role in counteracting themass transfer resistance of the dye molecules between the aqueous phaseand the solid phase. The increase in dye concentration serves as adriving force to overcome the resistance, increasing the probability ofcollision between the dye molecules and the adsorbent and leading tohigher adsorption (See S. S. Baral, N. Das, G. Roy Chaudhury, S. N. Das,A preliminary study on the adsorptive removal of Cr(VI) using seaweed,Hydrilla verticillata, J. Hazard. Mater., 171 (2009) 358-369,incorporated herein by reference in its entirety). Referring to FIG. 14,increasing the initial concentration of MB from 5 mg/L to 100 mg/L ledto an increase in the amount of MB adsorbed on the adsorbent. Referringto FIG. 15, by contrast, increasing the initial concentration of RhBfrom 5 mg/L to 75 mg/L led to an increase in the amount of RhB adsorbedon the adsorbent, however, a further increase in the initialconcentration from 75 mg/L to 100 mg/L led to a decrease in the amountof RhB adsorbed on the adsorbent. In both studies presented in FIG. 14and FIG. 15, the adsorption was performed at pH 7 and with 0.3 g of theadsorbent. While not intending to be limited or bound in any way bytheory, it is believed that at higher dye concentrations, the chance offormation of dye aggregates on the membrane may increase, resulting infewer vacant adsorption sites on the membrane. A high molecular weightdye, e.g. RhB, may have a stronger negative effect of theconcentration-induced aggregation on the adsorption capacity of themembrane than a low molecular weight dye, e.g. MB.

Example 4 Effect of the Adsorbent Dosage on the Amount of Dye Adsorbedor Removed Per Unit Mass of the Adsorbent Membrane and the DyeAdsorption/Removal Efficiency of the Adsorbent

To determine the effect of the adsorbent dosage on the amount of the dyeadsorbed on the adsorbent (membrane), a specific amount of the adsorbentwas added to 25 ml of a dye solution with an initial concentration of 50mg/L or 100 mg/L of MB or RhB. More specifically, in the study presentedin FIG. 16, the initial concentration of MB was 50 mg/L. In the studypresented in FIG. 17, the initial concentration of RhB was 50 mg/L. Inthe study presented in FIG. 18, the initial concentration of MB was 100mg/L. In the study presented in FIG. 19, the initial concentration ofRhB was 100 mg/L.

Referring to FIG. 16, line (a), FIG. 17, line (a), FIG. 18, line (a),and FIG. 19, line (a), the MB or RhB removal efficiency increased withan increase in the adsorbent dosage from 0.1 g to 0.3 g. As the dosageof the adsorbent was increased, a greater number of the adsorption siteswere generated on the adsorbent membrane, enabling more dye molecules tobe adsorbed, hence resulting in a higher removal efficiency.Additionally, at neutral pH (pH 7), the membrane was negatively chargeddue to the deprotonation of the carboxyl groups on the membrane. Withthe increase in the dosage, an electrostatic interaction between thenegatively charged membrane and the cationic dye molecules was alsoincreased. The combined effect of more adsorption sites and the strongerelectrostatic interaction led to the highest MB and RhB removalefficiencies with 0.3 g of the adsorbent as compared to those with 0.1 gor 0.2 g of the adsorbent. However, with the increase in the adsorbentdosage, the amount of the dye adsorbed per unit mass of the adsorbentmembrane was reduced from 58.03 to 15.02 mg/g for MB (which had aninitial concentration of 50 mg/L) shown in FIG. 16, line (b), and from49.96 to 15.36 mg/g for RhB (which had an initial concentration of 50mg/L) shown in FIG. 17, line (b). As shown in FIG. 18, line (b) and FIG.19, line (b), where both MB and RhB had an initial concentration of 100mg/L, there was a similar trend of the reduced amount of MB and RhBadsorbed per unit mass of the adsorbent membrane with the increasedadsorbent (membrane) dosage. A greater number of unsaturated sites at ahigher adsorbent dose decreased the adsorbate density on the membrane,as reflected in the lower q_(e) values, i.e. the amount of the dyeadsorbed per unit mass of the adsorbent (membrane) at equilibrium (SeeV. Vadivelan, K. V. Kumar, Equilibrium, kinetics, mechanism, and processdesign for the sorption of methylene blue onto rice husk, J. ColloidInterface Sci., 286 (2005) 90-100, incorporated herein by reference inits entirety).

Example 5 Effect of pH on the Dye Adsorption/Removal Efficiency of theAdsorbent

Cationic dyes exist in cationic form in an aqueous solution. The pH ofthe solution influences the charge on the adsorbent membrane which inturns affects the degree of adsorption of the dyes on the membrane. ThepH also influences the degree of ionization of the dye molecules. Sincethe membrane comprises HA/SA/HEC with carboxyl and phenolic groups, itwill exhibit different behaviors in acidic and basic media. At a low pH,i.e at a pH less than P_(zc), the cationic dye molecules are incompetition with the hydrogen ions to interact with the adsorbentmembrane. Moreover, the membrane may become positively charged becauseof the protonation of the carboxyl groups in addition to the H⁺ ions itadsorbs, repelling the positively charged MB and RhB molecules itcontacts. Owing to this combined effect, adsorption is low at a pH lessthan P_(zc). In general, there is an increase in the dye adsorptionefficiency with an increase in pH, because at a high pH the adsorptionsites become deprotonated, making the membrane negatively charged andattract the positively charged dye.

In both studies presented in FIG. 20 and FIG. 21, the amount of theadsorbent used was 0.3 g. Referring to FIG. 20, the adsorptionefficiency of the adsorbent for MB was greater at pH 9 than that at pH1, pH 4, or pH 7. Since the deprotonation of the carboxyl groups occursat pH>4, and the deprotonation of the phenolic OH groups occurs at pH>8,both the carboxylic and the phenolic groups are completely deprotonatedat pH 9, making the adsorbent membrane more negative at pH 9 than at pH1, pH 4, or pH 7 (See C. Dong, W. Chen, C. Liu, Y. Liu, H. Liu,Synthesis of magnetic chitosan nanoparticle and its adsorption propertyfor humic acid from aqueous solution, Colloids and Surfaces A:Physicochemical and Engineering Aspects, 446 (2014) 179-189,incorporated herein by reference in its entirety).

Referring to FIG. 21, by contrast, the adsorption efficiency for RhB wasthe least at pH 9 compared to that at pH 1, pH 4, or pH 7. In an aqueoussolution, RhB exists as a zwitterion at a pH of greater than 4 as shownin FIG. 22 (See L. Peng, P. Qin, M. Lei, Q. Zeng, H. Song, J. Yang, J.Shao, B. Liao, J. Gu, Modifying Fe3O4 nanoparticles with humic acid forremoval of Rhodamine B in water, J. Hazard. Mater., 209-210 (2012)193-198, incorporated herein by reference in its entirety). Thezwitterionic form of RhB reduces the interaction between the negativelycharged membrane and RhB, resulting in the decreased RhB adsorptionefficiency at a high pH, particularly at pH 9.

Example 6 Effect of Adsorption Time on the Dye Adsorption/RemovalEfficiency of the Adsorbent

Referring to FIGS. 23 and 24, the adsorbent adsorbed/removed most of theMB or RhB dye during the first 40 to 50 min of the adsorption process.More specifically, the adsorbent removed nearly 80% of the dye duringthe first 10 min of the adsorption process. Referring to FIG. 23, theadsorption was performed with the initial MB concentration of 50 mg/Land 0.3 g of the adsorbent and at pH 7. The adsorption equilibrium wasachieved for MB by 120 min after the start of the adsorption process,with the adsorbent removing 96% of MB from the MB containing solution.Referring to FIG. 24, the adsorption was performed with the initial RhBconcentration of 50 mg/L and 0.3 g of the adsorbent and at pH 7. Theadsorption equilibrium was achieved for RhB by 180 min after the startof the adsorption process, with the adsorbent removing 94.9% of RhB fromthe RhB containing solution. The high dye removal efficiency seen at theinitial stage of the adsorption process was a result of the presence ofexcess vacant sites on the membrane readily available for the dyemolecules to occupy. As the adsorption progressed and the availableadsorption sites on the membrane became fewer, it became difficult forthe dye molecules to be adsorbed, as indicated by the slower removal ofthe dye from the solution. The adsorption equilibrium is reached whenthe amount of dye adsorbed onto the adsorbent is equal to the amount ofdye desorbed from the adsorbent (See G. L. Dotto, L. A. A. Pinto,Adsorption of food dyes onto chitosan: Optimization process and kinetic,Carbohydr. Polym., 84 (2011) 231-238, incorporated herein by referencein its entirety).

Example 7 Determination of the Adsorption Isotherm for the Cationic DyeAdsorption by the Adsorbent Membrane

Information regarding how the adsorbate molecules distribute themselvesbetween the liquid and solid phase is generally provided by theadsorption isotherms (See T. S. Natarajan, H. C. Bajaj, R. J. Tayade,Preferential adsorption behavior of methylene blue dye onto surfacehydroxyl group enriched TiO2 nanotube and its photocatalyticregeneration, J. Colloid Interface Sci., 433 (2014) 104-114,incorporated herein by reference in its entirety). The dye adsorptiononto the adsorbent membrane was studied by fitting the adsorption datawith four isotherm models, i.e. the Langmuir, Fruendlich, Temkin andDubinin-Radushkevich equations.

1. Langmuir Isotherm

Langmuir isotherm is based on the assumption that all the adsorbentsites are equivalent and there is no interaction between the adsorbatemolecules forming a monolayer on the adsorbent surface. The linear formof the Langmuir equation is shown in Equation (3)

$\begin{matrix}{\frac{C_{e}}{q_{e}} = {\frac{1}{{bq}_{\max}} + \frac{C_{e}}{q_{\max}}}} & (3)\end{matrix}$

where C_(e) (mg/L) and q_(e) (mg/g) are the concentration and amount ofdye adsorbed per unit mass of the adsorbent at equilibrium,respectively; b is the Langmuir coefficient (L/mg) related to theaffinity of binding site; and q_(max) is the maximum adsorption capacityper unit mass of the adsorbent (mg/g).

2. Freundlich Isotherm

The Freundlich isotherm is applied to a heterogeneous adsorbent surfacebased on the assumption that the adsorbent sites are not equivalent.Equation (4) is the linear form of the Freundlich equation:

$\begin{matrix}{{\ln \; q_{e}} = {{\ln \; K_{F}} + {\frac{1}{n}\ln \; C_{e}}}} & (4)\end{matrix}$

where C_(e) (mg/L) and q_(e) (mg/g) are the concentration and amount ofdye adsorbed per unit mass of the adsorbent at equilibrium,respectively; K_(F) (mg^(1-1/n) L^(1/n) g⁻¹) and n are Freundlichcoefficients related to adsorption capacity and adsorption intensity,respectively. If the reciprocal of the Freundlich coefficient (1/n) isless than 1, it is considered as an indication of favorable adsorption.

3. Temkin Isotherm

The Temkin model takes into consideration the effects of an interactionbetween adsorbates and gives an idea about the heat of the adsorptionprocess. It has the equation

q _(e) =BlnA+BlnC _(e)  (5)

where,

$B = \frac{RT}{b}$

where R is the universal gas constant (8.314 J mol⁻¹K⁻¹); B and b (Jmol⁻¹) are Temkin coefficients and A (L/mg) is the equilibrium bindingconstant corresponding to maximum binding energy.

4. Dubinin-Radushkevich Isotherm

The Dubinin-Radushkevich (D-R) isotherm helps in determining whether anadsorption process is of a physical, an ion exchange or a chemicaladsorption type. It has the equation

lng _(e) =lnq _(max) −K _(D)ε²  (6)

where K_(D) (mol²/J²) is the D-R constant which is related to adsorptionenergy; ε is the Polanyi potential (J/mol) calculated with the followingequation:

$ɛ = {{RT}\; {\ln \left( {1 + \frac{1}{C_{e}}} \right)}}$

where R is the gas constant (J mol⁻¹ K⁻¹) is the absolute temperature(K) and C_(e) (mg/L) is the concentration of dye at equilibrium. The D-Rconstant is used to calculate mean free energy of adsorption E accordingto the following equation:

$E = \frac{1}{\sqrt{2\; K_{D}}}$

Depending on the value of E, the type of an adsorption process can beidentified. If E is <8 kJ/mol, a physical adsorption may be dominant. IfE lies between 8 kJ/mol and 16 kJ/mol, the adsorption process may be anion exchange adsorption; when E is >16 kJ/mol, the adsorption processmay be a chemical adsorption.

Referring to Table 1 showing the adsorption isotherm results of all themodels, the adsorption data fitted the Dubinin-Radushkevich isothermmore satisfactorily than the other isotherms. The regressioncoefficients for the Langmuir, Freundlich and Temkin isotherms werelower than that for the D-R model. The mean free energy of adsorption(E) calculated from the D-R model was less than 8 kJ/mol for both of thedyes, indicating that the adsorbent membrane adsorbs the dye moleculesthrough physical adsorption.

TABLE 1 Isotherm parameters for MB and RhB adsorption on the HA/SA/HECmembrane Dye MB RhB Langmuir Isotherm q_(max) 2.85 (mg/g) 18.814 (mg/g)b 0.193 (L/mg) 0.293 (L/mg) R² 0.623 0.96 Freundlich Isotherm 1/n 0.950.485 K_(F) 1.404 4.11 R² 0.517 0.55 Temkin Isotherm B 7.62 3.744 A0.819 (L/mg) 4.58 (L/mg) R² 0.827 0.787 Dubinin- Radushkevich IsothermK_(D) 3.548 × 10⁻⁶ (mol²/J²) 6.02 × 10⁻⁷ (mol²/J²) E 0.375 kJ/mol 0.91kJ/mol R² 0.954 0.967

Example 8 Determination of the Kinetic Model for the Cationic DyeAdsorption by the Adsorbent Membrane

The rate of an adsorption process depends upon the nature and propertiesof the adsorbent and the experimental conditions. The progress of thedye adsorption was examined by fitting the experimental data using sixdifferent kinetic models, with the results presented in Table 2. A highregression coefficient value (R²) (approaching unity) indicates theeffectiveness of the model in describing the kinetics of the dyeadsorption by the adsorbent membrane.

TABLE 2 Adsorption kinetic parameters for MB and RhB adsorption on theHA/SA/HEC membrane Methylene blue Rhodamine B q_(e,experimental) 15.39(mg/g) 14.839 (mg/g) Experimental parameters C₀ = 50 mg/L, pH 7, T = 27°C. Pseudo-first order kinetic model q_(e,calculated) 2.28 (mg/g) 3.14(mg/g) k₁ 3.3 × 10⁻² (min⁻¹) 3.9 × 10⁻² (min⁻¹) R² 0.786 0.964 Simplefirst order model k₁′ −1.1 × 10⁻² (min⁻¹) −1.9 × 10⁻³ (min⁻¹) R² 0.750.873 Pseudo-second order kinetic model q_(e,calculated) 15.625 (mg/g)14.97 (mg/g) k₂ 4.04 × 10⁻² (g/mg · min⁻¹) 3.17 × 10⁻² (g/mg · min⁻¹) R²0.9996 0.9997 Ritchie's second order kinetic model k₂′ 0.489 (min⁻¹)0.555 (min⁻¹) R² 0.912 0.925 Intraparticle diffusion model k_(id) 0.1908(mg/g · min^(−0.5)) 0.3574 (mg/g · min^(−0.5)) Intercept c 13.46 11.88R² 0.9587 0.9457 Elovich Model R² 0.95 0.934 β 1.74 (g/mg) 1.183 (g/mg)α 1.99 × 10⁹ 3.21 × 10⁵

1. Pseudo-First Order Kinetic/Lagergren Model

Equation (7) is the pseudo-first order kinetic/Lagergren model equation,

$\begin{matrix}{{\log \left( {q_{e} - q_{t}} \right)} = {{\log \; q_{e}} - \frac{k_{1}t}{2.303}}} & (7)\end{matrix}$

where q_(e) is the amount of dye adsorbed per unit mass of the adsorbentat equilibrium (mg/g), q_(t) is the amount of dye adsorbed per unit massof the adsorbent at time t (mg/g), and k₁ is the first order rateconstant (min⁻¹). The rate constant k₁ and q_(e) values were determinedfrom the slope and intercept of the linear graph of log(q_(e)−q_(t))versus time t, respectively. Lagergren's first order kinetic equation isusually applicable for the initial 30 to 50 min of an adsorption process(See Y. Wang, Y. Mu, Q.-B. Zhao, H.-Q. Yu, Isotherms, kinetics andthermodynamics of dye biosorption by anaerobic sludge, Sep. Purif.Technol., 50 (2006) 1-7, incorporated herein by reference in itsentirety). It is not suitable for the entire contact time of anadsorption process (See M. Otero, F. Rozada, L. F. Calvo, A. I. Garcia,A. Moran, Kinetic and equilibrium modelling of the methylene blueremoval from solution by adsorbent materials produced from sewagesludges, Biochemical Engineering Journal, 15 (2003) 59-68, incorporatedherein by reference in its entirety). Referring to FIGS. 25 and 26, andTable 2, fitting the experimental data with this model resulted in aconsiderable deviation of the experimental values from the theoreticalvalues, with the R² value of 0.786 for MB adsorption and of 0.96 for RhBadsorption. Moreover, the calculated q_(e) values for the MB and RhBadsorption did not agree well with the experimental q_(e) values,indicating that the pseudo-first order model was not best suited torepresent the dye adsorption process.

2. Simple First Order Kinetic Model

Equation (8) is the simple first order kinetic model equation:

$\begin{matrix}{{\log \; C_{t}} = {{\frac{k_{1^{\prime}}}{2.303}t} + {\log \; C_{0}}}} & (8)\end{matrix}$

where C_(t) and C₀ are the dye concentrations at time t and time 0(t=0). k_(1′) is the rate constant (min⁻¹) obtained from slope of thelinear graph of log C_(t) versus t. Since the concentration of dyedecreased with time, the slope of the linear graph of log C_(t) versus twas negative, resulting in a negative rate constant value. Referring toFIGS. 27 and 28, and Table 2, the regression coefficient was the leastwith this model amongst all the models studied, suggesting that thismodel is not applicable to the adsorption data.

3. Pseudo-Second Order Kinetic Model

Equation (9) is the pseudo-second order kinetic model equation:

$\begin{matrix}{\frac{t}{q_{t}} = {\frac{1}{k_{2}q_{e}^{2}} + \frac{t}{q_{e}}}} & (9)\end{matrix}$

where q_(e) and q_(t) in mg/g are the amount of dye adsorbed per unitmass of the adsorbent at equilibrium and at time t, respectively, k₂ isthe second order rate constant (g/mg·min⁻¹) calculated from theintercept of the linear graph of t/q_(t) versus time t. For most of theadsorbate-adsorbent systems, particularly when the adsorbate is apollutant, the rate of the adsorption is best represented by thepseudo-second order kinetic model (See X. Zhang, P. Zhang, Z. Wu, L.Zhang, G. Zeng, C. Zhou, Adsorption of methylene blue onto humicacid-coated Fe3O4 nanoparticles, Colloids and Surfaces A:Physicochemical and Engineering Aspects, 435 (2013) 85-90, incorporatedherein by reference in its entirety). The pseudo-second order kineticmodel was developed by Ho to describe chemisorption involving covalentforces or ion exchange as valency forces between an adsorbent and anadsorbate (See Y. S. Ho, Review of second-order models for adsorptionsystems, J. Hazard. Mater., 136 (2006) 681-689, incorporated herein byreference in its entirety). The model allows determination of theadsorption capacity, the rate constant and the initial rate ofadsorption without knowing any parameter in advance. Referring to Table2 and FIGS. 29 and 30, the correlation coefficient (R²) for the lineargraph of (t/q_(t)) versus t was close to unity (>0.99) for both MB andRhB adsorption. Moreover, the calculated q_(e) value was in goodagreement with the experimentally determined q_(e) value, suggestingthat the second order kinetic model best explained the kinetics of theMB and RhB adsorption on the HA/SA/HEC membrane.

4. Ritchie's Second Order Kinetic Model

Equation (10) is Ritchie's second order kinetic model equation:

$\begin{matrix}{{\frac{q_{e}}{q_{e} - q_{t}} = {1 + k_{2}}},t} & (10)\end{matrix}$

where k_(2′) is the rate constant (min⁻¹) and can be obtained byplotting q_(e)/(q_(e)−q_(t)) versus t. Ritchie proposed this model as analternative to Elovich model on the assumption that adsorption ratedepends solely on the fraction of unoccupied sites at any time t (See A.G. Ritchie, Alternative to the Elovich equation for the kinetics ofadsorption of gases on solids, Journal of the Chemical Society, FaradayTransactions 1: Physical Chemistry in Condensed Phases, 73 (1977)1650-1653, incorporated herein by reference in its entirety). Referringto Table 2 and FIGS. 31 and 32, this model was not suitable to representthe kinetics of the adsorption of MB or RhB by the adsorbent based onthe R² values and a considerable deviation of the experimental data fromthe calculated data.

5. Intraparticle Diffusion Model

Whether the adsorption of the dye molecules onto the membrane involvesan intraparticle diffusion process was studied using the Webber andMoris plot according to Equation (11)

q _(t) =k _(id) t ^(0.5) +c  (11)

where k_(id) is the intraparticle diffusion rate constant (mg·g⁻¹min^(−0.5)) obtained from the slope of the linear graph of q_(t) versust^(0.5) (See N. Nasuha, B. H. Hameed, A. T. Din, Rejected tea as apotential low-cost adsorbent for the removal of methylene blue, J.Hazard. Mater., 175 (2010) 126-132, incorporated herein by reference inits entirety). The rate controlling step in the adsorption process maybe one or more of the following steps (i) boundary layer diffusion whichis due to external surface adsorption of dye molecules, (ii)intraparticle diffusion underlying the gradual adsorption process and(iii) final equilibrium stage (See W. H. Cheung, Y. S. Szeto, G. McKay,Intraparticle diffusion processes during acid dye adsorption ontochitosan, Bioresour. Technol., 98 (2007) 2897-2904, incorporated hereinby reference in its entirety). Referring to FIGS. 33 and 34, it isevident that the dye adsorption onto the membrane occurred in threesteps as mentioned above. The first part of the graph showed a steepincrease in q_(t) value indicating a fast diffusion of the dye moleculesfrom the bulk of the solution to the external boundary of the adsorbent.The second part of the graph was the rate limiting step involvingdiffusion from the boundary layer to the interior or the active sites ofthe adsorbent. The final plateau region was due to a very slowadsorption rate at equilibrium. It is known that if the plot q_(t)versus t^(0.5) passes through the origin, then only the intraparticlediffusion is the rate limiting process. However in the currentinvestigation, the presence of the intercept, which is directlyproportional to the boundary layer thickness, indicates the presence ofthe boundary effect (See M. Dogan, M. Alkan, A. Turkyilmaz, Y. Ozdemir,Kinetics and mechanism of removal of methylene blue by adsorption ontoperlite, J. Hazard. Mater., 109 (2004) 141-148, incorporated herein byreference in its entirety). Thus for MB and RhB adsorption onto themembrane, intraparticle diffusion and surface adsorption are therate-limiting steps.

6. Elovich Model

In recent years, the Elovich model is used to describe the kinetics ofadsorption of pollutants from aqueous solutions. Elovich model isapplied to heterogeneous adsorbing systems involving chemisorption withthe assumption that the adsorption rate decreases with time due to anincreased coverage of the adsorbent surface with the adsorbates (See G.Zhao, J. Li, X. Wang, Kinetic and thermodynamic study of 1-naphtholadsorption from aqueous solution to sulfonated graphene nanosheets,Chem. Eng. J. (Lausanne), 173 (2011) 185-190, incorporated herein byreference in its entirety). Equation (12) is the simplified Elovichequation with the boundary conditions to be applied:

$\begin{matrix}{q_{t} = {{\frac{1}{\beta}{\ln ({\alpha\beta})}} + {\frac{1}{\beta}{\ln (t)}}}} & (12)\end{matrix}$

where q_(t) is the amount of dye adsorbed at time t, α is the initialrate of adsorption (mg/g·min), and β (g/mg) is the desorption constantrelated to the activation energy of chemisorption and indicates thenumber of sites available for adsorption (See R. Jayakumar, M.Rajasimman, C. Karthikeyan, Sorption of hexavalent chromium from aqueoussolution using marine green algae Halimeda gracilis: Optimization,equilibrium, kinetic, thermodynamic and desorption studies, Journal ofEnvironmental Chemical Engineering, 2 (2014) 1261-1274, incorporatedherein by reference in its entirety).

Referring to FIGS. 35 and 36 and Table 2, the fitting of theexperimental data with the Elovich model was satisfactory with theregression coefficients of 0.95 and 0.934 for MB and RhB adsorption,respectively. The initial rate of adsorption (a) was much higher for MBadsorption than for RhB adsorption.

Amongst all the kinetic models, the pseudo-second order kinetic modelbest characterized the dynamics of the MB and RhB adsorption by theadsorbent membrane.

Example 9 Effect of Temperature on the Amount of MB and RhB Adsorbed PerUnit Mass of the Adsorbent Membrane

Referring to FIGS. 37 and 38, the amount of MB or RhB adsorbed per unitmass of the adsorbent membrane at any time point during the adsorptionprocess increased with the increasing temperature from 35° C. to 55° C.,indicating that the adsorption process was endothermic in nature. Theheat energy may assist the dye molecules in overcoming the activationbarrier to attach themselves to the adsorbent membrane. Theintraparticle diffusion rate of the dye to the membrane pores may alsoincrease with the increase in temperature.

Example 10 Regeneration and Reusability of the Adsorbent Membrane

An adsorption process using an adsorbent that can be regenerated andreused is more environmentally friendly and economically viable.Regeneration and reusability of the prepared adsorbent membrane for thedye adsorption was investigated by subjecting the adsorbent to fourcycles of adsorption and desorption. The adsorption was performed bycontacting the adsorbent with a dye solution containing 50 mg/L of MB orRhB at pH 7. The desorption was performed by treating the dye-adsorbedadsorbent with a 0.1 M HCl solution. Referring to FIG. 39, there was nosignificant reduction in the dye removal efficiency of the adsorbentafter one, two, three, or four cycles of adsorption and desorption, withthe removal efficiency being maintained at nearly 98% for both MB andRhB after each successive cycle.

1. A method of removing at least one cationic dye from an aqueoussolution, comprising: contacting the aqueous solution with an adsorbentcomprising a water-insoluble membrane disposed on a substrate, whereinthe water-insoluble membrane comprises cross-linked humic acid, at leastone alginate, and hydroxyethyl cellulose, and wherein the contactingforms a treated aqueous solution having a lower concentration of the atleast one cationic dye relative to the aqueous solution.
 2. The methodof claim 1, wherein the weight ratio of humic acid:at least onealginate:hydroxyethyl cellulose lies in the range 5-30:40-90:5-30. 3.The method of claim 1, wherein the at least one alginate comprises anunmodified alginate, a modified alginate, or a combination thereof. 4.The method of claim 1, wherein the humic acid, the at least onealginate, and the hydroxyethyl cellulose are cross-linked by at leastone cross-linking agent selected from the group consisting of aldehydes,oxidoreductase enzymes, and a combination thereof.
 5. The method ofclaim 1, wherein at least one of the cross-linked humic acid, at leastone alginate, and hydroxyethyl cellulose comprises or is modified tocomprise a first molecular moiety, and the substrate comprises or ismodified to comprise a second molecular moiety, and wherein thedisposition of the water-insoluble membrane on the substrate comprisesbinding of the first molecular moiety to the second molecular moiety. 6.The method of claim 1, wherein the pH of the aqueous solution rangesfrom about 3 to about
 10. 7. The method of claim 1, wherein thesubstrate comprises at least one selected from the group consisting ofpolypropylene, polystyrene, PET (polyethylene terephthalate), polyimide,PEN (polyethylene naphthalate), agarose, acetate cellulose, PC(polycarbonate), glass, plastic, rubber, a metal, an alloy, a ceramic, acarbonaceous material, a polymer, sand, silicon, and silica.
 8. Themethod of claim 1, wherein the at least one cationic dye is selectedfrom the group consisting of methylene blue, rhodamine B, crystalviolet, basic fuchsin safranin, pararosaniline, and a combinationthereof.
 9. The method of claim 8, wherein the at least one cationic dyeis methylene blue, and wherein the adsorbent removes at least 90% of themethylene blue from the aqueous solution.
 10. The method of claim 8,wherein the at least one cationic dye is rhodamine B, and wherein theadsorbent removes at least 90% of the rhodamine B from the aqueoussolution.
 11. The method of claim 8, wherein the at least one cationicdye is rhodamine B, and wherein the concentration of the rhodamine B inthe aqueous solution is adjusted to be below 75 mg/L.
 12. The method ofclaim 1, further comprising regenerating the adsorption ability of theadsorbent.
 13. The method of claim 12, wherein the regenerating theadsorption ability of the adsorbent comprises treating the adsorbentwith at least one mineral or strong acid and/or at least one organicacid for a period of time effective to desorb the at least one cationicdye from the adsorbent.
 14. The method of claim 1, wherein theconcentration of the water-insoluble membrane of the adsorbentcontacting the aqueous solution ranges from 0.04 g/100 ml to 0.64 g/100ml of the aqueous solution.
 15. The method of claim 1, wherein theadsorbent is disposed in a fixed bed reactor or fluidized bed reactorand the contacting involves passing the aqueous solution through thefixed bed reactor or fluidized bed reactor.
 16. The method of claim 15,wherein the fixed bed reactor comprises a cartridge.
 17. The method ofclaim 16, wherein the cartridge further comprises activated carbon. 18.The method of claim 1, wherein the adsorbent has a form selected fromthe group consisting of a granule, a pellet, a sphere, a powder, a wovenfabric, a non-woven fabric, a mat, a felt, a block, and a honeycomb. 19.The method of claim 1, wherein the aqueous solution is contacted withthe adsorbent at a temperature of about 10-90° C. and a pressure ofabout 1-50 bar.
 20. The method of claim 1, further comprising removingthe adsorbent from the treated aqueous solution.